1. Discharge Studies in MPGD: what could be done in the frame of WG-2 collaboration P. Fonte, V. Peskov

2. WG-2 tasks (from the RD-51 proposal)

3. Discharges in MPGDs

4. I) In bad quality detectors – imperfections II) In good quality detectors - there are several fundamental reasons: 1) Raether limit 2) Rate effect 3) Jets 4) Feedbacks 5) Surface streamers Want cause the breakdowns?

5. Imperfections: Usually cause persisting discharges at the fixed place in the detector

6. a) b) c) d) Ideal holes Holes with sharp edges Holes with debris or dust particles Holes with areas of slightly conductive surfaces (dirt) - + Cathode Anode Examples for hole-type gas amplifiers: Common « standards »: before comparing maximum achievable gains one have to verify that the discharges are randomly distributed over the detector surface

7. 1)Raether limit

8. At A max n 0 ≥Q max =10 8 electrons an avalanche transits to a spark. A max n 0 =10 8 is called a Raether limit. Discharges in parallel-plate geometry

9. Raether limit fro MPGDs: It was recently discovered  that a similar limit applies for every micropattern detectors: GEMs, MICROMEGAS and others: A max n 0 =Q max =10 6 -10 7 electrons, where n 0 is the number of primary electrons created in the drift region of the detector (Q max depends on the detector geometry and the gas composition) (  see Y. Ivanchenkov et al., NIM A422,1999,300 and V. Peskov et al., IEEE Nucl. Sci. 48, 2001, 1070)

10. Conclusions: With single primary electrons gains up to 10 6 - 10 7 in principle are possible With 55 Fe (n 0 ~230 electrons) the maximum achievable gain is <10 5 With alphas (n 0 =105) the maximum achievable gain <100 This was well observed in the case of MPGDs

11. MPGD CERTIFICATION DETECTO R MAX GAIN MAX CHAR GE MSGC2000 4 10 7 ADV PASS MSGC 1000 2 10 7 MICROW ELL 2200 4.4 10 7 MICROME GAS 3000 6 10 7 GEM2000 4 10 7 The maximum gain before discharge is almost the same for all MPGD tested: S. Bachmann et al, Nucl. Instr. and Meth. A479(2002)294 ~3000 ~2000 MEASURE GAIN WITH 55 Fe X-RAYS AND DISCHARGE PROBABILITY WITH INTERNAL ALPHA SOURCE FROM 220 Rn MICROMEGAS GEM F. Sauli, Report at the RD51 collaboration meeting in Amsterdam, 2008

12. Maximal gains with UV are 100 times higher than with X-rays. For UV and x-ray gun: The current in the plateau region (500-750V) was the same: 0.1nA. The maximum current in gain measurements was always kept below 0.5nA 55 Fe NEW Pulse-mode (~1kHz) Cu X-ray gun, current-mode Single-THGEM : Ar+5%CH 4 WIS old pulse-mode UV Current-mode NEW 10 4 THGEM geometry: Holes dia: 0.5 mm Pitch: 1 mm Thickness: 0.8 mm Rim: 0.1mm A. Breskin, V. Peskov et al, Report at the RD51 meeting in Paris, 2008

13. What was established up to now is just a general picture Detailed studies are still needed: a) Simulations b) Geometry and gas optimization

14. Geometrical optimization?

15. Regions with parallel fields lines where any streamer, if appear, is unquenched and may reach the cathode Why there are sparks in micropattern gaseous detectors? Because there are regions with parallel field lines, so streamers develop there by the same mechanism as in PPAC

16. Transition to streamer occurs when An 0 ≥Q max =10 8 electros Self-quenched streamer Strimers give huge amplitudes but the are not harmful as well Streamers cannot propagate to the cathode because the electric field drops as 1/r Streamer

17. Signal’s amplitude in proportional and streamer modes For details see: P. Fonte et al., INFN Insrum. Bull, SLAC-Journal ICFA-15-1, 1997

18. The main designs of micropattern gaseous detectors Microstrip gaseous detectors Microdot gaseous detectors MICROMEGAS GEM 25-100μm 25μm 140μm

19. Empirical way to increase the Raethet limit: multistep detectors Raethre limit increases due to the diffusion effect?

20. Gas optimization?

21. Single-THGEM: Ne UV, current-mode 55 Fe Pulse-mode The maximum gains with x-rays in Ne are higher than in Ar+5%CH 4. In Ne breakdown voltages with UV and X-rays are closer. 10 4 THGEM geometry: Holes dia: 0.5 mm Pitch: 1 mm Thickness: 0.8 mm Rim: 0.1mm 10 4 A. Breskin, V. Peskov et al, Report at the RD51 meeting in Paris, 2008

22. Single-THGEM: Ne + CH 4 Same as with Ne: maximum gains with x-rays in Ne+CH 4 are higher than in Ar+5%CH 4 and breakdown voltages with UV and X-rays are close. 55Fe Pulse-mode 55Fe Pulse-mode UV Current-mode UV Current-mode THGEM geometry: Holes dia: 0.5 mm Pitch: 1 mm Thickness: 0.8 mm Rim: 0.1mm 10 4 A. Breskin, V. Peskov et al, Report at the RD51 meeting in Paris, 2008

23. A possible interpretation: - Raether limit: established in large-gap avalanche detectors but valid for MPGDs (Ivanchenkov NIM A 1999), though may be different - A*n 0 =10 6 -10 7 electrons where A is the maximum achievable gain, n0-number of primary electrons deposited by the radiation in the drift region  X-rays: different gain compared to UV - In Ne/CH 4 Raether limit possibly differs from Ar/CH 4 due to ~ 5-fold longer range of 55 Fe photoelectrons (~1mm), resulting in lower ioinization density per “hole”.

24. More details…

25. Raether limit for PPAC and MICROMEGAS is reached at n 0 >50 electrons V. Peskov et al., IEEE Nucl. Sci. 48,2001,1070 PPAC Raether limit MICROMEGAS Raether limit

26. For n 0 >50 electrons “Rather” limit works well, however for n 0 <20 electrons other factor starts dominating like field emission from sharp edges, gain fluctuation… PPAC Raether limit Single GEM Raether limit..similar for GEM-type detectors V. Peskov et al., IEEE Nucl. Sci. 48,2001,1070

27. Proposed “common standards" in discharge studies and comparisons ●Discharges should be randomly distributes over the detector surface. ● For Raether limit verification use: UV light, 55 Fe and alphas ● Measure not only discharge rate vs. applied voltage, but the discharge energy and evaluate the destructive effect (some detector may die after one sparks others withstand hundred of sparks)

28. 2) Rate effect

29. A max Parallel plate detector (PPAC) Signal amplitude does not drop with rate, however there is a rate limit for each amplitude Amplitudes P. Fonte et al IEEE Nucl. Sci 46,1999,321

30. A max For each micropattern detector the amplitude remains unchanged with rate, however the maximum achievable gain drops with rate Rate limit of micropattern gaseous detectors Amplitudes P. Fonte et al, NIM A419,1998,405

31. Common “standards”: When reporting rate indices sparks one have to mention at what gas gain this was measured/observed

32. 3) Jets

33. Electron jets: P. Fonte et al., IEEE Nuc. Sci 46,1999,321

36. Other evidences: Hysteresis: one cannot apply the voltage immediately after the breakdown (“Memory effect” well documented in the case of RPC and Compass RICH Depends on gas This effect should be studied as well

37. 4) Feedbacks (essential for detector operating in noble gases or combined with photocathodes) A f γ=1(A f γ + =1 or A f γ ph =1)-”slow” mechanism of discharges The probabilities γ + and γ ph are increasing with the increasing the photocathode QE and it’s sensitivity to visible light and with electric field near the cathode

38. 5) Surface streamers

39. HV Amplifier Surface streamer V. Peskov et al., NIM A397,1997, 243

40. Discharges “prevention:”  Increase or “bypass” the Raether limit( gas optimization, detector geometry optimization/multistep approach)  Reduce feedbacks (when it is essential)  Any other measures..? Not too much…

41. Examples:  Resistive GEMs  Strip electrodes terminated on resistors ( V. Peskov et al report at IEEE Nucl Sci, Dresden 2008)  MICROMEGAs with resistive coating (see Van Der Graaf presentation) Spark-proofed MPGDs

42. Optimization of the RPC electrodes resistivity

43. Instead of conclusions: ●A lot of work is required to better understand discharges and protection against the discharges ● We can try to identify today who is interested to participate in these studies and how ● One of the possible way is to cluster these studies around CERN-Coimbra –Weizmann institute where this activity was already started and enforce the local teams with visitors ● Any other suggestion are very welcome

44. Spairs

45. Possible discussion topics at WG-2 meeting at CERN: 1. Raether limit for micropattern detectors: a)Experimental evidence, simulations, possible ways of its increasing (gases, geometry) b)Rate induced breakdowns: Avalanche overlapping and Raether limit 2. Cathode excitation effect at low and high counting rates : a) Hysteresis in breakdown voltage b) Long-term discharge memory effect c)Jets of electrons 3. Common “standards”: a) Speaks measurements (distinguish sparks due to defect from sparks due to the Raether limit) How we compare sparks: spark energy, sparks rate How we evaluate spark damages (after how many spars the detector dies?)? b)Gain measurements: ionization chamber vs. charge injection methode c) Gain stability (due to the charging up effect and dielectric polarization): Low rate High rate Short-term stability Long-term stability d) Quantum efficiency measurements for photosensitive micropattern detectors- what should be a common reference: TMAE, calibrated detectors, Cherenkov light?